1. Field of the Invention
The present invention relates to a non-aqueous electrolyte battery separator comprising a heat-resistant nitrogen-containing aromatic polymer and a ceramic powder, and a lithium secondary battery.
2. Description of the Related Art
A lithium primary battery or lithium secondary battery using a non-aqueous electrolyte is highly useful due to its property that high capacity and high energy density are obtained. As a main structure of these batteries, a separator composed of an electrically insulating porous film is intercalated between positive and negative electrodes, an electrolyte solution containing a lithium salt dissolved is impregnated into pores of the film, and the positive electrode and negative electrode and the separator are laminated, or wound in the form of a roll. It is required for a lithium secondary battery to make various safe measures against problems derived from its high capacity and high energy density, for example, large increase in battery temperature because of short-circuit inside and outside of a battery. For solving such problems, there have been made various ideas for the separator.
Particularly, as a safety precaution to which a separator can contribute, shut down property and short circuit property are under notice. Herein, the term shut down (also referred to as fuse) means that battery temperature increases by troubles such as overcharge, external or internal short, a part of a separator is melted to obstruct pores, and current is cut off, and the temperature in this phenomenon is called shut down temperature. The term short circuit means that temperature further increases from the shut down temperature, the separator is melted and a large hole is formed to cause short again, and the temperature in this phenomenon is called short circuit temperature. Decrease in the shut down temperature and increase in short circuit temperature are required for a non-aqueous electrolyte battery separator.
Conventionally, as a separator of a lithium secondary battery, a thin porous film is used, and for example, Celgard (registered trade mark) manufactured by Hoechst Co. is preferably used as a separator of a lithium secondary battery. However, a non-aqueous electrolyte battery separator having further excellent heat-resistance and higher short circuit temperature has been desired.
Regarding a raw material of such a non-aqueous electrolyte battery separator, use of a whole aromatic polyamide-based polymer having excellent heat-resistance has been investigated. For example, Japanese Patent Application Publication (JP-B) No. 59-36939 describes a method for producing a porous film made of an aromatic polymer, that is aromatic polyamide or aromatic polyimide, JP-B No. 59-14494 describes that a method for producing a porous film made of an aromatic polyamide and that it can be used as a battery separator. Further, Japanese Patent Application Laid-Open (JP-A) No. 5-335005 describes use of Normex (registered trade mark) paper (meta-aramid paper) manufactured by du Pont Co. as a separator of a lithium secondary battery. Likewise, JP-A Nos. 7-78608 and 7-37571 also suggest use of non-woven fabric or paper-like sheet made of meta-aramid as a battery separator. Further, JP-A Nos. 62-37871 and 2-46649 describes use of polyimide as a separator of a non-aqueous electrolyte solution battery. In these publications, a material is employed which exhibits excellent ion permeability and battery property while maintaining heat-resistance.
On the other hand, regarding shut down and short circuit, JP-A No. 3-291848 and JP-B No. 4-1692 suggest, for securing safety of a battery when short is occurred inside or outside of a battery, that a battery separator is allowed to have shut down function which cut off current, by providing an obstruction material which can be heat-melted on a porous film made of a thermoplastic resin and by covering the surface of a micro porous film by heat-melting of this obstruction material. Further, JP-A Nos. 60-52 and 60-136161 suggest that a battery separator is allowed to have shut down function, by adhering a polyethylene-based resin powder onto polypropylene non-woven fabric and by heat-melting the resin to obstruct pores of the non-woven fabric. However, in these suggestions, thermoplastic resins are used, therefore, heat-resistance is not sufficient and short circuit temperature is low, and use is restricted in view of safety.
The present inventors have intensively investigated a separator which does not have problems as described above, and found that a separator containing ceramic powder in a heat-resistant nitrogen-containing aromatic polymer is highly heat-resistant and has high short circuit temperature, and further has excellent ion permeability.
An object of the present invention is to provide a non-aqueous electrolyte battery separator having excellent ion permeability and battery property while maintaining merits of a heat-resistant nitrogen-containing aromatic polymer that heat-resistance is high and short circuit temperature is high.
Another object of the present invention is to provide a non-aqueous electrolyte battery separator which has such safety that shut down occurs in over-heating, has so high short circuit temperature that it is not melted when heated, and further has excellent safety. Further object of the present invention is to provide a lithium secondary battery having high short-circuit temperature and excellent safety by using such as separator.
Namely, the present invention relates to (1) a non-aqueous electrolyte battery separator comprising a heat-resistant nitrogen-containing aromatic polymer and a ceramic powder.
Further, the present invention relates to (2) a non-aqueous electrolyte battery separator comprising a heat-resistant nitrogen-containing aromatic polymer, a ceramic powder, and a substrate made of woven fabric, non-woven fabric, paper or porous film.
Further, the present invention relates to (3) a non-aqueous electrolyte battery separator according to (1) or (2), wherein the non-aqueous electrolyte battery separator contains a thermoplastic polymer which is melted at a temperature of 260xc2x0 C. or less in an amount of 10% by weight or more based on the whole separator, and said thermoplastic polymer is melted when temperature increases and obstructs pore of said separator.
Moreover, the present invention relates to (4) a non-aqueous electrolyte battery separator comprising a coated film obtained by a method comprising the following steps of:
(a) preparing a slurry solution which may contain a thermoplastic resin which is melted at a temperature of 260xc2x0 C. or less, by dispersing a ceramic powder in a solution of a polar organic solvent containing a heat-resistant nitrogen-containing aromatic polymer in an amount of 1 to 1500 parts by weight based on 100 parts by weight of said heat-resistant nitrogen-containing aromatic polymer,
(b) producing a coated film by coating said slurry solution,
(c) depositing said heat-resistant nitrogen-containing aromatic polymer on said coated film,
(d) removing the polar organic solvent from said coated film, and
(e) drying said coated film.
Furthermore, the present invention relates to (5) a non-aqueous electrolyte battery separator comprising a coated film obtained by a method comprising the following steps of:
(a) preparing a slurry solution which may contain a thermoplastic resin which is melted at a temperature of 260xc2x0 C. or less, by dispersing a ceramic powder in a solution of a polar organic solvent containing a heat-resistant nitrogen-containing aromatic polymer in an amount of 1 to 1500 parts by weight based on 100 parts by weight of said heat-resistant nitrogen-containing aromatic polymer,
(b) producing a coated film by coating said slurry solution on a substrate made of woven fabric, non-woven fabric, paper or porous film,
(c) depositing said heat-resistant nitrogen-containing aromatic polymer on said coated film,
(d) removing the polar organic solvent from said coated film, and
(e) drying said coated film.
Also, the present invention relates to (6) a lithium secondary battery comprising a non-aqueous electrolyte battery separator of any of the inventions (1) to (5).
The non-aqueous electrolyte battery separator of the present invention comprises a heat-resistant nitrogen-containing aromatic polymer and a ceramic powder.
The heat-resistant nitrogen-containing aromatic polymer of the present invention is a polymer containing a nitrogen atom and an aromatic ring in the backbone, and examples thereof include an aromatic polyamide (hereinafter, may sometimes be referred to as xe2x80x9caramidxe2x80x9d), aromatic polyimide (hereinafter, may sometimes be referred to as xe2x80x9cpolyimidexe2x80x9d), aromatic polyamideimide and the like.
Examples of the aramid include a meta-oriented aromatic polyamide ((hereinafter, may sometimes be referred to as xe2x80x9cmeta-aramidxe2x80x9d) and para-oriented aromatic polyamide (hereinafter, may sometimes be referred to as xe2x80x9cpara-aramidxe2x80x9d), and a para-aramid is preferable since it tends to become porous.
The para-amide is obtained by polycondensation of a para-oriented aromatic diamine with a para-oriented aromatic dicarboxylic halide, and substantially consists essentially of repeating units in which amide bonds are bonded in para-orientation or corresponding orientation (for example, orientation extending co-axially or in parallel to reverse direction such as 4,4xe2x80x2-biphenylene, 1,5-naphthalene, 2,6-naphthalene and the like).
Specifically, there are exemplified para-aramids having structure of para-orientation or orientation corresponding to para-orientation, such as poly(p-phenyleneterephthalamide), poly(p-benzamide), poly(4,4xe2x80x2-benzanilideterephthalamide), poly(p-phenylene-4,4xe2x80x2-biphenylenedicarboxylic amide), poly(p-phenylene-2,6-naphthalenedicarboxylic amide), poly(2-chloro-p-phenyleneterephthalamide), p-phenyleneterephthalamide/2,6-dichloro p-phenyleneterephthalamide copolymer and the like.
The para-aramid of the present invention can be dissolved in polar organic solvent to prepare a solution having low viscosity, and has an intrinsic viscosity preferably of 1.0 dl/g to 2.8 dl/g, further preferably of 1.7 dl/g to 2.5 dl/g for excellent coating property. A satisfactory film strength may not be obtained when the intrinsic viscosity is less than 1.0 dl/g. When the intrinsic viscosity is over than 2.8 dl/g, a stable para-amide solution may not be easily obtained, and it may be difficult to form a film because a para-amide is deposited.
The polar organic solvent herein used is for example a polar amide-based solvent or polar urea-based solvent, and specific examples thereof include N,N-dimethylformamide, N,N-dimethylacetoamide, N-methyl-2-pyrrolidone, tetramethylurea and the like, but are not limited to these examples.
The para-aramid of the present invention is preferably a porous polymer in the form of fibril. The fibril-like polymer is microscopically in the form of non-woven cloth and in the form of a porous layer containing pore and forms a so-called para-aramid porous resin.
The polyimide used in the present invention is not particularly restricted and preferably a whole aromatic polyimide produced by polycondensation of an aromatic diacid anhydride with a diamine. Specific examples of the diacid anhydride include pyromellitic dianhydride, 3,3xe2x80x2-4,4xe2x80x2-diphenylsulfonetetracarboxylic dianhydride, 3,3xe2x80x2-4,4xe2x80x2-benzophenonetetracarboxylic dianhydride, 2,2xe2x80x2-bis(3,4-dicarboxyphenyl)hexafluoropropane, 3,3xe2x80x2-4,4xe2x80x2-biphenyltetracarboxylic dianhydride and the like, but are not limited to these examples. Specific examples of the diamne include oxydianiline, p-phenylenediamine, benzophenonediamine, 3,3xe2x80x2-methylenedianiline, 3,3xe2x80x2-diaminobenzophenone, 3,3xe2x80x2-diaminodiphenylsulfone, 1,5xe2x80x2-naphthalenediamine and the like, but are not limited to these examples. In the present invention, when a porous film is made directly from a polyimide solution, a polyimide soluble in a solvent can be suitably used. As this polyimide, there is for example listed a polyimide, that is a polycondensate of 3,3xe2x80x2-4,4xe2x80x2-diphenylsulfonetetracarboxylic dianhydride with an aromatic diamine.
As the polar organic solvent used in the polyimide, dimethylsulfoxide, cresol, o-chlorophenol and the like can be suitably used in addition to those exemplified for the aramid.
In the present invention, a porous polyimide is preferable. Example, a solid film can be subjected to boring by mechanical process, laser process and the like to be made into porous material. When a polyimide film is made by a solution casting method, a porous film can be produced by controlling molding conditions of the polyimide such as polymer concentration in coating and the like. Further, a uniform and fine porous film can be produced using a solution having any polymer concentration by combining ceramic powders. Also, gas permeability can be controlled by added amount of a ceramic powder.
The non-aqueous electrolyte battery separator of the present invention is required to contain a ceramic powder. The ceramic powder is entangled and arrested by a heat-resistant nitrogen-containing aromatic polymer, and situated being totally or partially dispersed in the non-aqueous electrolyte battery separator.
The ceramic powder used in the present invention has an average particle size of the primary particle of preferably 1.0 xcexcm or less, more preferably 0.5 xcexcm or less in view of influence on strength of a non-aqueous electrolyte battery separator and smoothness on coated surface. The average particle size of the primary particle is measured by analyzing a photograph made by an electron microscope with a particle size measuring apparatus. When the average particle size of the primary particle of a ceramic powder is over 1.0 xcexcm, the separator may become fragile and also coated surface may become fragile. The content of the ceramic powder is preferably from 1% by weight to 95% by weight, more preferably from 5% by weight to 50% by weight based on the weight of the non-aqueous electrolyte battery separator. When the content of the ceramic powder is less than 1% by weight based on the weight of the non-aqueous electrolyte battery separator, effect for promoting ion permeability and battery property may not be sufficient, and when over 95% by weight, the separator may become fragile and handling thereof may become difficult. The form of the ceramic powder is not particularly restricted, and sphere and random forms can also be used.
As the raw material of the ceramic powder in the present invention, metal oxide, metal nitride, metal carbide and the like having electrically insulating property are listed, and for example, alumina, silica, titanium dioxide, zirconium oxide and the like are preferably used.
As the substrate of the present invention, there are listed porous woven fabric, non-woven fabric, paper and porous films made of electrically insulating organic, inorganic fiber or pulp. Among them, non-woven fabric, paper or porous films are preferable in view of cost and small thickness.
The raw material of said substrate may be organic or inorganic, synthetic or natural material providing it has electrically insulating property, and materials containing organic fiber and/or inorganic fiber and/or organic fiber pulp and/or inorganic fiber pulp are listed. Specifically, natural fiber such as a fiber comprising a Manila hemp, thermoplastic polymer fiber and the like are listed as the organic fiber. As the fiber comprising a thermoplastic polymer, fibers such as polyolefins like polyethylene, polypropylene and the like, rayon, vinylon, polyester, acryl, polystyrene, nylon and the like are listed. As the inorganic fiber, glass fiber, alumina fiber and the like are listed.
The non-aqueous electrolyte battery separator (2) of the present invention comprises a heat-resistant nitrogen-containing aromatic polymer, a ceramic powder and a substrate made of woven fabric, non-woven fabric, paper or porous film.
The non-aqueous electrolyte battery separator (2) of the present invention is preferably one in which the substrate is coated with the above-described heat-resistant nitrogen-containing aromatic polymer containing a ceramic powder, or pore of the substrate is filled with the above-described heat-resistant nitrogen-containing aromatic polymer, or the substrate is coated with the above-described heat-resistant nitrogen-containing aromatic polymer and pore of the substrate is filled with the above-described heat-resistant nitrogen-containing aromatic polymer.
When used in the non-aqueous electrolyte battery separator (2) of the present invention, the weight per unit area of a substrate is preferably 40 g/m2 or less, more preferably 15 g/m2 or less. The pore percentage of the substrate is preferably 40% or more, further preferably 50% or more. The thickness of the substrate is preferably 70 xcexcm or less, further preferably 25 xcexcm or less.
The non-aqueous electrolyte battery separator (3) of the present invention contains a thermoplastic polymer which is melted at a temperature of 260xc2x0 C. or less in an amount of 10% by weight or more, preferably 30% by weight or more, more preferably 40% by weight or more based on the whole separator, and the thermoplastic polymer is melted when temperature increases to obstruct pore of the separator. The above-described thermoplastic polymer may advantageously be a polymer which is melted in temperature increase, if it is used as a battery separator. The thermoplastic polymer is preferably a polymer which is melted at a temperature of 260xc2x0 C. or less, more preferably 200xc2x0 C. or less in view of shut down function, when it is used as a separator in a lithium secondary battery. The melting temperature is preferably about 100xc2x0 C. or more because it is suitable for shut down.
As the thermoplastic polymer, polyolefin resins, acrylic resins, styrene resins, polyester resins, nylon resins and the like are listed. In particular, polyethylenes such as low density polyethylene, high density polyethylene, linear polyethylene and the like, or low molecular weight wax components thereof, or polyolefin resins such as polypropylene and the like are suitably used since they have an appropriate melting temperature and are available easily. These may be used alone or in a mixture thereof.
The thermoplastic polymer used in the present invention is preferably a powder having an average particle size of preferably 10 xcexcm or less, more preferably 6 xcexcm or less in view of dispersibility into a solvent and smoothness of the coated surface. The form of the powder particle is not particularly restricted, and any of spherical and random forms can be used.
In the non-aqueous electrolyte battery separator (3) of the present invention, a thermoplastic polymer is totally or partially dispersed in the form of a particle in a non-aqueous electrolyte battery separator, and the dispersion embodiment is not particularly restricted providing the thermoplastic polymer is melted when temperature increases to block pore of the separator.
The non-aqueous electrolyte battery separator (4) of the present invention comprises a coated film obtained by a method comprising the following steps (a) to (e) of:
(a) preparing a slurry solution which may contain a thermoplastic resin which is melted at a temperature of 260xc2x0 C. or less, by dispersing a ceramic powder in a solution of a polar organic solvent containing a heat-resistant nitrogen-containing aromatic polymer in an amount of 1 to 1500 parts by weight based on 100 parts by weight of said heat-resistant nitrogen-containing aromatic polymer,
(b) producing a coated film by coating said slurry solution,
(c) depositing said heat-resistant nitrogen-containing aromatic polymer on said coated film,
(d) removing the polar organic solvent from said coated film, and
(e) drying said coated film.
The method for producing a non-aqueous electrolyte battery separator (4) of the present invention will be specifically described.
Step (a): Preparation of Slurry Solution
Cases using a para-aramid and a polyimide as a heat-resistant nitrogen-containing aromatic polymer will be excemplified.
When a para-aramid is used, for example, in a polar organic solvent containing 2 to 10% by weight of an alkaline metal chloride or alkaline earth metal chloride dissolved, a para-oriented aromatic dicarboxylic dihalide is added in an amount of 0.94 to 0.99 mol per 1.00 mol of a para-oriented aromatic diamine and is poly-condensed at a temperature of xe2x88x9220 to 50xc2x0 C. to prepare a polar organic solvent solution of an aramid in which the concentration of the produced para-oriented aromatic polyamide is from 1 to 10% (by weight) and the intrinsic viscosity is from 1.0 to 2.8 dl/g.
As the para-oriented aromatic diamine used in polycondensation of a para-aramid, p-phenylenediamine, 4,4xe2x80x2-diaminobiphenyl, 2-methyl-p-phenylenediamine, 2-chloro-p-phenylenediamine, 2,6-dichloro-p-phenylenediamine, 2,6-naphthalenediamine, 1,5-naphthalenediamine, 4,4xe2x80x2-diaminobenzanilide, 3,4xe2x80x2-diaminodiphenyl ether and the like are listed. The para-oriented aromatic diamines can be used alone or in a mixture thereof and subjected to polycondensation.
As the para-oriented aromatic dicarboxylic dihalide used in polycondensation of a para-aramid, terephthalic dichloride, biphenyl-4,4xe2x80x2-diarboxylic dichloride, 2-chloroterephthalic dichloride, 2,5-dichloroterephthalic dichloride, 2-methylterephthalic dichloride, 2,6-naphthalenedicarboxylid dichloride, 1,5-naphthalenedicarboxylid dichloride, and the like are listed. The para-oriented aromatic dicarboxylic dihalide can be used alone or in a mixture thereof and subjected to polycondensation.
For the purposed of improving solubility of a para-aramid into a solvent, an alkaline metal chloride or alkaline earth metal chloride is preferably used. Specific examples include, lithium chloride and calcium chloride, but are not limited to these examples.
The added amount of the above-described chloride into the polymer system is preferably in the range from 0.5 to 6.0 mol, more preferably in the range from 1.0 to 4.0 mol based on 1.0 mol of an amide group produced in polycondensation. When the amount of the chloride is 0.5 mol or less, the solubility of a para-aramid produced may become insufficient, and when the amount is over 6.0 mol, it substantially may exceed the amount dissolved of the chloride into a solvent.
In general, when the amount of an alkaline metal chloride or alkaline earth metal chloride is less than 2% by weight, solubility of a para-aramid may be insufficient, and when over 10% by weight, an alkaline metal chloride or alkaline earth metal chloride may not be dissolved in a polar organic solvent such as a polar amide-based solvent, polar urea-based solvent or the like.
When the para-amide concentration is 0.5% by weight or less, productivity remarkably may decrease to cause industrial disadvantage. When the amount of a para-aramid is over 10% by weight, the para-aramid may be deposited and a stable para-aramid solution may not be easily obtained.
As the polar organic solvent solution of a polyimide, for example, N-methyl-2-pyrrolidone solution of a polyimide in which imidation has been completed is listed. N-methyl-2-pyrrolidone solution is prepared by polycondensation reaction of 3,3xe2x80x2,4,4xe2x80x2-diphenylsulfonetetracarboxylic dianhydride with an aromatic diamine such as 4,4xe2x80x2-bis(p-aminophenoxy)diphenylsulfone and the like. When a polyimide is used as this polar organic solvent, cresol, o-chlorophenol and the like are listed in addition to the above-exemplified compounds.
A ceramic powder is dissolved in the above-described polar organic solvent solution in an amount of 1 to 1500 parts by weight, preferably 5 to 100 parts by weight per 100 parts by weight of a heat-resistant nitrogen-containing aromatic polymer. When the amount of the ceramic powder less than 1 parts by weight, improvement of in ion permeability and battery property is not sufficient. When the amount over 1500 parts by weight, the separator becomes fragile and handling thereof becomes difficult. optionally, a thermoplastic polymer may be added to the slurry solution.
Step (b): Production of a Coated Film
This slurry solution is coated on a base film, steel belt, roll, drum and the like to form a wet coated film.
As the base film, there are listed, for example, polyethylene terephthalate, paper subjected to releasing treatment, and the like. It is industrially often conducted to coat a solution on a steel belt having corrosion resistance which has been subjected to mirror finish. In small scale system, it can also be possible to coat a solution on a roll or a drum having corrosion resistance which has been subjected to mirror finish.
As the coating method, coating methods such as knife coating, blade coating, bar coating, gravure coating, die coating and the like are listed. In small scale system, bar coating, knife coating and the like are convenient. However, industrially, die coating is preferable in which a solution is not brought in contact with outer air.
Step (c): Deposition of Heat-resistant Nitrogen-containing Aromatic Polymer
The resulted coated film is placed in atmosphere controlled under constant humidity at a temperature of preferably 20xc2x0 C. or more, and a heat-resistant nitrogen-containing aromatic polymer is allowed to deposit, then, immersed in a coagulation solution. Alternatively, it is immersed in a coagulation solution, deposition and coagulation of a polymer are conducted simultaneously to obtain a wet coated film. For uniform and quick coagulation, it is also possible that a poor solvent, for example, water or the like is previously added to a slurry solution, to prepare deposited condition.
In the case of a para-aramid, it is also possible that a part or all of a solvent is evaporated and simultaneously a polymer is deposited, namely, solvent removal process and deposition process are conducted simultaneously to obtain a semi-dried or dried coated film.
As the coagulation solution, an aqueous solution, alcoholic solution or the like may advantageously be used. Though the solution is not particularly limited, it is preferable to use an aqueous solution or alcoholic solution containing a polar organic solvent since solvent removal process can be industrially simplified.
Step (d): Removal of Polar Organic Solvent
Then, a polar organic solvent is removed from this coated film on which a heat-resistant nitrogen-containing aromatic polymer is deposited. For this removal, a part or all of the polar organic solvent may be evaporated, or it may be removed by using a solvent which can dissolve the polar organic solvent such as water, aqueous solution, alcoholic solution or the like. When the removal is conducted using water, it is preferable to use ion-exchanged water. Further it is also industrially preferable that washing is conducted in an aqueous solution containing the polar organic solvent in certain amount, then, washing with water is conducted. For drying, a solvent used for washing is evaporated by heating to be removed. When a thermoplastic polymer which is melted is contained, the drying temperature in this procedure is preferably not more than the temperature for the melting.
When, a para-aramid is prepared using an alkaline metal chloride or alkaline earth metal chloride, the alkaline metal chloride or alkaline earth metal chloride is washed and removed together with a solvent from a wet coated film on which the para-aramid has been deposited. Alternatively, the alkaline metal chloride or alkaline earth metal chloride is washed and removed from a dried coated film. For this removal, there is adopted a method in which a coated film is immersed in a solution and a solvent and a chloride are eluted. As the solution for eluting a solvent or a chloride, water, aqueous solution or alcoholic solution is preferable since it can dissolve both of the solvent and the chloride.
(e): Drying
The coated film from which a polar organic solvent has been removed can be preferably dried at the melting temperature or less of a polymer which is heat-melted to produce an intended dried coated film.
This dried coated film can be used as a non-aqueous electrolyte battery separator without any other treatment. For imparting shut down property, it is preferable to contain a thermoplastic polymer, this thermoplastic polymer may be added in any step. Also, it is preferable that a fine particle-like suspension of the thermoplastic polymer is coated on the dried coated film and dried to form a fine particle layer of the thermoplastic resin.
As the coating method, coating methods such as knife coating, blade coating, bar coating, gravure coating, die coating and the like are listed. In small scale system, bar coating, knife coating and the like are convenient.
The non-aqueous electrolyte battery separator (5) of the present invention comprises a coated film obtained by a method comprising the following steps (a) to (e) of:
(a) preparing a slurry solution which may contain a thermoplastic resin which is melted at a temperature of 260xc2x0 C. or less, by dispersing a ceramic powder in a solution of a polar organic solvent containing a heat-resistant nitrogen-containing aromatic polymer in an amount of 1 to 1500 parts by weight based on 100 parts by weight of said heat-resistant nitrogen-containing aromatic polymer,
(b) producing a coated film by coating said slurry solution on a substrate made of woven fabric, non-woven fabric, paper or porous film,
(c) depositing said heat-resistant nitrogen-containing aromatic polymer on said coated film,
(d) removing the polar organic solvent from said coated film, and
(e) drying said coated film.
The method for producing a non-aqueous electrolyte battery separator (5) of the present invention will be specifically described.
The method for producing a non-aqueous electrolyte battery separator (5) of the present invention is in the same manner as for the separator (4) of the present invention except that a substrate made of woven fabric, non-woven fabric, paper or porous film is used.
The step (a) is the same as the step (a) for the separator (4) of the present invention.
The step (b) is the same as the step (b) for the separator (4) of the present invention except that coating is conducted on a substrate made of woven fabric, non-woven fabric, paper or porous film. Alternatively, it is also permissible that a slurry solution is coated on a roll or drum, then, the substrate is mounted to be impregnated with the solution.
The steps (c), (d) and (e). can be carried out in the same manners as in the steps (c), (d) and (e) for the separator (4) of the present invention.
This dried coated film can be used as a non-aqueous electrolyte battery separator without any other treatment. For imparting or reinforcing shut down property, it is preferable to contain a thermoplastic polymer, this thermoplastic polymer may be added in any step. Also, it is preferable that a fine particle-like suspension of the thermoplastic polymer is coated on the dried coated film and dried to form a fine particle layer of the thermoplastic resin. The coating method is the same as that for the separator (4) of the present invention.
The thickness of the non-aqueous electrolyte battery separator of the present invention is preferably from 5 to 100 xcexcm. When the thickness is less than 5 xcexcm, strength as a non-aqueous electrolyte battery separator may lack and handling thereof may be difficult. As a non-aqueous electrolyte battery separator, higher thickness provides easier handling, however, a separator having so smaller thickness as not to cause short circuit is desirable for making internal resistance as low as possible in the case of a lithium secondary battery thought there is no strict restriction regarding thickness in the case of a nickel-cadmium battery. That is, in a lithium secondary battery separator, the thickness is preferably from 5 to 100 xcexcm, further preferably from 5 to 50 xcexcm, particularly preferably from 5 to 30 xcexcm.
It is known that the heat-resistant nitrogen-containing aromatic polymer used in the present invention exhibits almost no degradation in strength at a temperature from room temperature to about 200xc2x0 C., and has excellent heat resistance. Further, it has self-extinguishing property, is not decomposed and keeps its shape up to about 300xc2x0 C., and heat-decomposed at a temperature over this range. Further, it is known that a ceramic powder exhibits almost no degradation in strength at a temperature up to about 1000xc2x0 C., and has excellent heat resistance. Therefore, in a non-aqueous electrolyte battery using the separator of the present invention, even if the battery temperature increases due to short circuit inside or outside of the battery and the like, shut down function works, and even if the temperature further increases, it keeps the shape until high temperature, that is, it keeps insulation property between positive and negative electrodes and manifests excellent safety.
Pore size of the non-aqueous electrolyte battery separator of the present invention, or a diameter of the sphere (hereinafter, sometimes referred to as pore size) when the void can be approximated to sphere, is preferably about 1 xcexcm or less. When the average size of the pore size is over 1 xcexcm, there is possibility of occurrence of such problems that when a carbon powder which is a main component of positive and negative electrodes or a fragment thereof drops, short circuit and the like tends to occur.
The non-aqueous electrolyte battery separator of the present invention can be suitably used in a lithium secondary battery. In the non-aqueous electrolyte battery separator of the present invention, film form of the non-aqueous electrolyte battery separator is maintained even when temperature increases since it contains a heat-resistant nitrogen-containing aromatic polymer and a ceramic powder. Further, in the case of the non-aqueous electrolyte battery separator of the present invention containing a thermoplastic resin, when the battery is locally or totally heated, the thermoplastic polymer is melted, and introduced into fine pores of the separator to obstruct the fine pores, preventing current. Further, even when temperature increases, the polymer does not flow out since it is introduced in the fine pores. Thus, shut down of the battery is accomplished.
The non-aqueous electrolyte battery separator of the present invention manifests excellent ion permeability and battery property while keeping the feature of a heat-resistant nitrogen-containing aromatic polymer that heat-resistance is high and short circuit temperature is high. Further, the non-aqueous electrolyte battery separator of the present invention has safety property that shut down occurs in overheat, and further, is not melted when heated and has high short circuit temperature, providing more excellent safety. Moreover, the lithium battery of the present invention has higher short circuit temperature and more excellent safety by using the above-described separator.